Light-emitting element, display module, lighting module, light-emitting device, display device, electronic appliance, and lighting device

ABSTRACT

A multicolor light-emitting element that utilizes fluorescence and phosphorescence and is advantageous for practical application is provided. The light-emitting element has a stacked-layer structure of a first light-emitting layer containing a host material and a fluorescent substance and a second light-emitting layer containing two kinds of organic compounds and a substance that can convert triplet excitation energy into luminescence. Note that light emitted from the first light-emitting layer has an emission peak on the shorter wavelength side than light emitted from the second light-emitting layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 15/341,053, filed Nov. 2, 2016, now allowed, which is a continuation of U.S. application Ser. No. 14/467,532, filed Aug. 25, 2014, now U.S. Pat. No. 9,515,279, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2013-174560 on Aug. 26, 2013, Serial No. 2013-249449 on Dec. 2, 2013, and Serial No. 2014-112119 on May 30, 2014, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a driving method thereof, or a fabrication method thereof. In particular, one embodiment of the present invention relates to a light-emitting element containing an organic compound as a light-emitting substance, a display module, a lighting module, a display device, a light-emitting device, an electronic appliance, and a lighting device.

BACKGROUND ART

In recent years, research and development of a light-emitting element (organic EL element) that uses an organic compound and utilizes electroluminescence (EL) have been actively promoted. In a basic structure of such a light-emitting element, an organic compound layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying voltage to the element, light can be emitted from the light-emitting substance.

The light-emitting element is a self-luminous element and thus has advantages such as high visibility and no need for backlight, and is considered to be suitable as a flat panel display element. In addition, it is also a great advantage that a display including the light-emitting element can be fabricated as a thin and lightweight display and has very fast response speed.

The light-emitting element can provide planar light emission; thus, a large-area element can be formed easily. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.

In the case of such an organic EL element, electrons from a cathode and holes from an anode are injected into an EL layer, so that current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is excited and provides light emission.

The excited state of an organic compound can be a singlet excited state (S*) or a triplet excited state (T*), and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S*:T*=1:3.

In a compound that emits light from the singlet excited state (hereinafter, referred to as fluorescent substance), at room temperature, generally light emission from the triplet excited state (phosphorescence) is not observed while only light emission from the singlet excited state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent substance is assumed to have a theoretical limit of 25% based on the ratio of S* to T* that is 1:3.

In contrast, in a compound that emits light from the triplet excited state (hereinafter, referred to as a phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Since intersystem crossing easily occurs in a phosphorescent compound, the internal quantum efficiency can be increased to 100% in theory. That is, a light-emitting element using a phosphorescent substance can easily have higher emission efficiency than a light-emitting element using a fluorescent substance. For this reason, light-emitting elements using phosphorescent substances are now under active development in order to obtain highly efficient light-emitting elements.

A white light-emitting element disclosed in Patent Document 1 includes a light-emitting region containing a plurality of kinds of light-emitting dopants that emit phosphorescence. An element disclosed in Patent Document 2 includes an intermediate layer (a charge-generation layer) between a fluorescent layer and a phosphorescent layer (i.e., the element is what is called a tandem element).

REFERENCE

-   Patent Document 1: Japanese Translation of PCT International     Application No. 2004-522276 -   Patent Document 2: Japanese Published Patent Application No.     2006-024791

DISCLOSURE OF INVENTION

As multicolor light-emitting elements typified by white light-emitting elements, elements including an intermediate layer (a charge-generation layer) between a fluorescent layer and a phosphorescent layer have been developed (Patent Document 2), and some of them have been put into practical application. In the light-emitting element having such a structure, light with a short wavelength is emitted from the fluorescent layer and light with a long wavelength is emitted from the phosphorescent layer.

In the structure, fluorescence is used as light with a short wavelength that has a problem in the lifetime and phosphorescence is used as light with a long wavelength. The structure is employed in order to achieve stable characteristics of a multicolor light-emitting element in spite of efficiency lower than that of an element in which phosphorescence is used as light with a long wavelength and light with a short wavelength.

The multicolor light-emitting element that has the above-described structure in which the reliability is put ahead of the performance is suitable for practical application as compared with general light-emitting elements, which still often have a problem in the lifetime; however, a larger number of films are formed in order to obtain one multicolor light-emitting element, which hinders the practical application.

There are some reasons for providing the intermediate layer between the phosphorescent layer and the fluorescent layer in the multicolor element having the structure. One of the reasons is for suppressing quenching of phosphorescence caused by the fluorescent layer.

In the fluorescent layer, a substance having a condensed aromatic ring (especially, a condensed aromatic hydrocarbon ring) skeleton, typified by anthracene, is generally used as a host material. The substances having the fused aromatic ring skeleton often have a relatively low triplet level. Thus, in the case where the fluorescent layer is formed in contact with a phosphorescent layer, the triplet excitation energy generated in the phosphorescent layer is transferred to the triplet level of the host material in the fluorescent layer to be quenched. Since a triplet exciton has a long lifetime, the diffusion length of the exciton is long and excitation energy generated in the phosphorescent layer as well as excitation energy generated at the interface between the fluorescent layer and the phosphorescent layer are quenched by the host material in the fluorescent layer. Thus, a significant reduction in emission efficiency is caused.

The above-described problems are solved by using a host material with high triple excitation energy for the fluorescent layer. In that case, however, the singlet excitation energy of the host material is higher than the triplet excitation energy, so that energy is not sufficiently transferred from the host material to a fluorescent dopant. This results in insufficient emission efficiency in the fluorescent layer. In addition, non-radiative decay of the host material is accelerated to degrade the characteristics (especially, lifetime) of the element. When the singlet excitation energy of the host material is higher than necessary, the HOMO-LUMO gap of the host material is large. This leads to an excessive increase in drive voltage.

In view of the above, an object of one embodiment of the present invention is to provide a light-emitting element that utilizes fluorescence and phosphorescence and is advantageous for practical application. Another object of one embodiment of the present invention is to provide a light-emitting element that utilizes fluorescence and phosphorescence, has a small number of fabrication steps owing to a relatively small number of layers to be formed, and is advantageous for practical application.

Another object of one embodiment of the present invention is to provide a light-emitting element that utilizes fluorescence and phosphorescence and has high emission efficiency.

Another object of one embodiment of the present invention is to provide a light-emitting element that utilizes fluorescence and phosphorescence, has a relatively small number of layers to be formed, is advantageous for practical application, and has high emission efficiency. Another object of one embodiment of the present invention is to provide a novel light-emitting element.

Another object of one embodiment of the present invention is to provide a display module, a lighting module, a light-emitting device, a display device, an electronic appliance, and a lighting device that can be fabricated at low cost by using the light-emitting element.

Another object of one embodiment of the present invention is to provide a display module, a lighting module, a light-emitting device, a display device, an electronic appliance, and a lighting device that have reduced power consumption by using the light-emitting element.

It is only necessary that at least one of the above-described objects be achieved in the present invention.

The above-described objects can be achieved by a light-emitting element that has a stacked-layer structure of a first light-emitting layer 113 a containing a host material and a fluorescent substance and a second light-emitting layer 113 b containing two kinds of organic compounds that form an exciplex and a substance that can convert triplet excitation energy into luminescence. Note that a light-emitting element in which light emitted from the first light-emitting layer 113 a has an emission spectrum peak on the shorter wavelength side than an emission spectrum peak of light emitted from the second light-emitting layer 113 b more effectively achieves the above-described objects.

One embodiment of the present invention is a light-emitting element that includes a pair of electrodes and an EL layer interposed between the pair of electrodes. The EL layer includes the first light-emitting layer and the second light-emitting layer. The emission spectrum of light emitted from the first light-emitting layer 113 a is located on the shorter wavelength side than the emission spectrum of light emitted from the second light-emitting layer. The first light-emitting layer contains at least a fluorescent substance and a host material. The second light-emitting layer contains at least a substance that can convert triplet excitation energy into luminescence, a first organic compound, and a second organic compound. The first organic compound and the second organic compound form an exciplex.

Another embodiment of the present invention is a light-emitting element that includes a pair of electrodes and an EL layer interposed between the pair of electrodes. The EL layer includes the first light-emitting layer and the second light-emitting layer that are stacked in contact with each other. The emission spectrum of light emitted from the first light-emitting layer is located on the shorter wavelength side than the emission spectrum of light emitted from the second light-emitting layer. The first light-emitting layer contains at least a fluorescent substance and a host material. The second light-emitting layer contains at least a substance that can convert triplet excitation energy into luminescence, a first organic compound, and a second organic compound. The first organic compound and the second organic compound form an exciplex.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which energy is transferred from the exciplex to the substance that can convert triplet excitation energy into luminescence.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the singlet excitation level of the host material is higher than the singlet excitation level of the fluorescent substance, and the triplet excitation level of the host material is lower than the triplet excitation level of the fluorescent substance.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the triplet excitation level of the host material is lower than the triplet excitation levels of the first organic compound and the second organic compound.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the host material has a condensed aromatic ring skeleton.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the host material has an anthracene skeleton.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the host material is an organic compound having an anthracene skeleton and the fluorescent substance is an organic compound having a pyrene skeleton.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the second light-emitting layer contains n (n is an integer of 2 or more) kinds of substances having different emission spectra as the substance that can convert triplet excitation energy into luminescence.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the second light-emitting layer includes n layers and the n layers contain different substances that can convert triplet excitation energy into luminescence.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the second light-emitting layer contains a first phosphorescent substance and a second phosphorescent substance that have different emission spectra as the substance that can convert triplet excitation energy into luminescence.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the first phosphorescent substance emits light in the red region, the second phosphorescent substance emits light in the green region, and the fluorescent substance emits light in the blue region.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the first phosphorescent substance has an emission spectrum peak in the range of 580 nm to 680 nm, the second phosphorescent substance has an emission spectrum peak in the range of 500 nm to 560 nm, and the fluorescent substance has an emission spectrum peak in the range of 400 nm to 480 nm.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the second light-emitting layer includes a first phosphorescent layer and a second phosphorescent layer, a first phosphorescent substance is contained in the first phosphorescent layer, and a second phosphorescent substance is contained in the second phosphorescent layer.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the first light-emitting layer, the first phosphorescent layer, and the second phosphorescent layer are stacked in this order.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the first light-emitting layer is formed on an anode side of the pair of electrodes and the second phosphorescent layer is formed on a cathode side of the pair of electrodes.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the first phosphorescent substance exhibits a carrier-trapping property in the first phosphorescent layer.

Another embodiment of the present invention is a light-emitting element having the above-described structure in which the carrier-trapping property is an electron-trapping property.

Another embodiment of the present invention is a display module that includes any of the above-described light-emitting elements.

Another embodiment of the present invention is a lighting module that includes any of the above-described light-emitting elements.

Another embodiment of the present invention is a light-emitting device that includes any of the above-described light-emitting elements and a unit for controlling the light-emitting element.

Another embodiment of the present invention is a display device that includes any of the above-described light-emitting elements in a display portion and a unit for controlling the light-emitting element.

Another embodiment of the present invention is a lighting device that includes any of the above-described light-emitting elements in a lighting portion and a unit for controlling the light-emitting element.

Another embodiment of the present invention is an electronic appliance that includes any of the above-described light-emitting elements.

Note that the light-emitting device in this specification includes, in its category, an image display device that uses a light-emitting element. The category of the light-emitting device in this specification includes a module in which a light-emitting element is provided with a connector such as an anisotropic conductive film or a tape carrier package (TCP); a module having a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip on glass (COG) method. Furthermore, the category includes a light-emitting device that is used in lighting equipment or the like.

In one embodiment of the present invention, a multicolor light-emitting element that utilizes fluorescence and phosphorescence, has a relatively small number of layers to be formed, and is advantageous for practical application can be provided.

In another embodiment of the present invention, a multicolor light-emitting element that utilizes fluorescence and phosphorescence and has high emission efficiency can be provided.

In another embodiment of the present invention, a multicolor light-emitting element that utilizes fluorescence and phosphorescence, has a relatively small number of layers to be formed, is advantageous for practical application, and has high emission efficiency can be provided.

In another embodiment of the present invention, a display module, a lighting module, a light-emitting device, a display device, an electronic appliance, and a lighting device that can be fabricated at low cost by using any of the above-described light-emitting elements can be provided.

In another embodiment of the present invention, a display module, a lighting module, a light-emitting device, a display device, an electronic appliance, and a lighting device that have reduced power consumption by using any of the above-described light-emitting elements can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of light-emitting elements.

FIGS. 2A and 2B are conceptual diagrams of an active matrix light-emitting device.

FIGS. 3A and 3B are conceptual diagrams of active matrix light-emitting devices.

FIG. 4 is a conceptual diagram of an active matrix light-emitting device.

FIGS. 5A and 5B are conceptual diagrams of a passive matrix light-emitting device.

FIGS. 6A and 6B illustrate a lighting device.

FIGS. 7A, 7B1, 7B2, 7C and 7D illustrate electronic appliances.

FIG. 8 illustrates a light source device.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic appliance.

FIG. 13 shows current density-luminance characteristics of a light-emitting element 1.

FIG. 14 shows luminance-current efficiency characteristics of the light-emitting element 1.

FIG. 15 shows voltage-luminance characteristics of the light-emitting element 1.

FIG. 16 shows luminance-external quantum efficiency characteristics of the light-emitting element 1.

FIG. 17 shows an emission spectrum of the light-emitting element 1.

FIG. 18 shows time dependence of normalized luminance of the light-emitting element 1.

FIGS. 19A and 19B show emission spectra of a light-emitting element 2 and a light-emitting element 3.

FIG. 20 shows an emission spectrum of a light-emitting element 4.

FIG. 21 shows a correlation between energy levels of substances and exciplexes in a light-emitting element of one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the invention should not be construed as being limited to the description in the following embodiments.

Embodiment 1

FIG. 1A is a diagram illustrating a light-emitting element of one embodiment of the present invention. The light-emitting element includes at least a pair of electrodes (a first electrode 101 and a second electrode 102) and an EL layer 103 including a light-emitting layer 113. The light-emitting layer 113 includes the first light-emitting layer 113 a and the second light-emitting layer 113 b.

FIG. 1A also illustrates a hole-injection layer 111, a hole-transport layer 112, an electron-transport layer 114, and an electron-injection layer 115 in the EL layer 103. However, this stacked-layer structure is an example, and the structure of the EL layer 103 in the light-emitting element of one embodiment of the present invention is not limited thereto. Note that in FIG. 1A, the first electrode 101 functions as an anode, and the second electrode 102 functions as a cathode.

The first light-emitting layer 113 a contains a fluorescent substance and a host material. The second light-emitting layer 113 b contains a first organic compound, a second organic compound, and a phosphorescent compound. In the light-emitting layer having the structure, a combination of the first organic compound and the second organic compound forms an exciplex.

This structure enables light originating from the fluorescent substance to be emitted efficiently from the first light-emitting layer 113 a and light originating from the phosphorescent substance to be emitted efficiently from the second light-emitting layer 113 b. Note that even when the light-emitting element does not include a charge-generation layer between the first light-emitting layer 113 a and the second light-emitting layer 113 b (i.e., even when the light-emitting element is not a tandem element), both fluorescence and phosphorescence can be obtained efficiently.

When a fluorescent layer and a phosphorescent layer are included in the same EL layer to emit light, the triplet excitation energy of the phosphorescent layer is generally transferred to a host material occupying a large part of the fluorescent layer. This causes a significant reduction in emission efficiency. The reason is as follows: since a substance having a condensed aromatic ring (especially, a condensed aromatic hydrocarbon ring) skeleton, which is typified by anthracene that has a low triplet level, is generally used as a host material, triplet excitation energy generated in the phosphorescent layer is transferred to the host material in the fluorescent layer, which results in non-radiative decay. At present, it is difficult to obtain a desired emission wavelength and favorable element characteristics or reliability without using a substance having a condensed aromatic ring skeleton in the fluorescent layer; thus, the structure in which the fluorescent layer and the phosphorescent layer are included in the same EL layer makes it difficult to obtain a light-emitting element having favorable characteristics.

Since a triplet excited state has a long relaxation time, the diffusion length of an exciton is long, many of the excitons generated in the phosphorescent layer are transferred to the fluorescent layer because of diffusion, and non-radiative decay of the excitons is caused. This further reduces the emission efficiency of the phosphorescent layer.

In this embodiment, the first organic compound and the second organic compound form an exciplex in the second light-emitting layer 113 b, and the triplet excitation energy is transferred from the exciplex to the phosphorescent substance, so that light emission can be obtained. This structure can solve the above-described problems.

An exciplex is an excited state formed from two kinds of substances. The two kinds of substances that have formed the exciplex return to a ground state by emitting light and serve as the original two kinds of substances. In other words, an exciplex itself does not have a ground state, and energy transfer between exciplexes or energy transfer to an exciplex from another substance is unlikely to occur accordingly in principle.

A process in which one of the first organic compound and the second organic compound as a cation and the other of the first organic compound and the second organic compound as an anion are adjacent to each other to form an exciplex (an electroplex process) is considered dominant for the generation of the exciplex in the light-emitting element. Even when one of the first organic compound and the second organic compound comes into an excited state, the one quickly interacts with the other of the first organic compound and the second organic compound to be adjacent to each other, so that an exciplex is formed; thus, most excitons in the second light-emitting layer 113 b exist as exciplexes.

The singlet excitation energy of an exciplex corresponds to a difference in energy between the lower HOMO level of one of the first organic compound and the second organic compound and the higher LUMO level of the other of the first organic compound and the second organic compound; thus, the singlet excitation energy of the exciplex is lower than the singlet excitation energy of both of the organic compounds, and singlet excitation energy transfer from the exciplex to the first organic compound and the second organic compound does not occur. Furthermore, the first organic compound and the second organic compound are selected so that the triplet excitation energy of the exciplex is lower than the triplet excitation energy of the first organic compound or the second organic compound, preferably lower than the triplet excitation energy of the first organic compound and the second organic compound, whereby energy transfer from an exciplex to the first organic compound and the second organic compound can hardly occur. In addition, energy transfer between the exciplexes hardly occurs as described above; thus, diffusion of the excitons in the second light-emitting layer 113 b hardly occurs. As a result, the above-described problems can be solved.

When the first light-emitting layer 113 a that is a fluorescent layer and the second light-emitting layer 113 b are in contact with each other, energy transfer (especially triplet energy transfer) from the exciplex to the host material of the first light-emitting layer 113 a can occur at the interface. However, diffusion of the excitons in the second light-emitting layer 113 b hardly occurs as described above; thus, energy transfer from the exciplex to the host material in the first light-emitting layer 113 a occurs in a limited area (i.e., the interface between the first light-emitting layer 113 a and the second light-emitting layer 113 b), and large loss of the excitation energy does not occur. Thus, one feature of one embodiment of the present invention is that high efficiency can be obtained even when the first light-emitting layer 113 a and the second light-emitting layer 113 b are in contact with each other although the light-emitting layers are not necessarily in contact with each other. In other words, an element structure in which the first light-emitting layer 113 a and the second light-emitting layer 113 b are in contact with each other is also one embodiment of the present invention.

Also in the case where the triplet excitation energy of the host material contained in the fluorescent layer is lower than the triplet excitation energy of the first organic compound and the second organic compound contained in the phosphorescent layer as described above, application of one embodiment of the present invention enables a light-emitting element to emit fluorescence and phosphorescence with high efficiency.

Furthermore, in the light-emitting element of one embodiment of the present invention, even when energy transfer (especially triplet energy transfer) from the exciplex to the host material in the first light-emitting layer 113 a or energy transfer from the phosphorescent substance to the host material in the first light-emitting layer 113 a occurs at the interface between the first light-emitting layer 113 a and the second light-emitting layer 113 b, the energy can be converted into luminescence in the first light-emitting layer 113 a. In other words, when the first light-emitting layer 113 a has a structure in which a singlet excited state is generated easily by triplet-triplet annihilation (T-T annihilation: TTA), the triplet excitation energy transferred from the exciplex to the host material at the interface can be converted into fluorescence in the first light-emitting layer 113 a. This enables energy loss of the light-emitting element of one embodiment of the present to be reduced. In order that the light-emitting layer 113 a can have the structure in which the single excited state is generated easily by TTA, it is preferable to select a host material and a fluorescent substance in the first light-emitting layer 113 a so that the singlet excitation level of the host material is higher than the singlet excitation level of the fluorescent substance and the triplet excitation level of the host material is lower than the triplet excitation level of the fluorescent substance. As a combination of the host material and the fluorescent substance that are in such a relation, a combination of a material having an anthracene skeleton as the host material and a material having a pyrene skeleton as the fluorescent substance, or the like is preferable.

Note that when the first light-emitting layer 113 a is too thick, emission from the second light-emitting layer 113 c is difficult to obtain. In addition, when the first light-emitting layer 113 a is too thin, emission from the first light-emitting layer 113 a is difficult to obtain. For those reasons, the thickness of the first light-emitting layer 113 a is preferably greater than or equal to 5 nm and less than or equal to 20 nm.

In the case where the first light-emitting layer 113 a is formed on the anode side, the first light-emitting layer 113 a preferably has a hole-transport property. In that case, a bipolar material having a high hole-transport property is preferably used. A material having an anthracene skeleton is preferable as such a material. Furthermore, when the fluorescent substance has a high hole-trapping property (e.g., a condensed aromatic amine compound described later), the concentration of the fluorescent substance is preferably lower than or equal to 5%, further preferably higher than or equal to 1% and lower than or equal to 4%, still further preferably higher than or equal to 1% and lower than or equal to 3%, in which case phosphorescence and fluorescence can be obtained in a balanced manner and with high efficiency. Note that the fluorescent substance exhibits a hole-trapping property when the HOMO level of the fluorescent substance is higher than the HOMO level of the host material.

Although there is no limitation on the combination of the first organic compound and the second organic compound in the second light-emitting layer 113 b as long as an exciplex can be formed, one organic compound is preferably a material having a hole-transport property and the other organic compound is preferably a material having an electron-transport property. In that case, a donor-acceptor excited state is formed easily, which allows an exciplex to be formed efficiently. In the case where the combination of the first organic compound and the second organic compound is a combination of the material having a hole-transport property and the material having an electron-transport property, the carrier balance can be controlled easily by adjusting the mixing ratio. Specifically, the weight ratio of the material having a hole-transport property to the material having an electron-transport property is preferably 1:9 to 9:1 inclusive. In order to increase quantum efficiency, it is particularly preferable that the weight ratio of the material having a hole-transport property to the material having an electron-transport property be 5:5 to 9:1 inclusive in a region closest to the anode in the second light-emitting layer 113 b. Since the carrier balance can be easily controlled in the light-emitting element having the above-described structure, a recombination region can also be easily adjusted. The light-emitting element of one embodiment of the present invention also has a feature in that an emission color can be adjusted by controlling the carrier balance as described above.

In the light-emitting element of this embodiment, a carrier recombination region is preferably distributed to some extent. For that, it is preferable that each light-emitting layer have a moderate degree of carrier-trapping property, and it is particularly preferable that the phosphorescent substance have an electron-trapping property. Examples of a substance that has a high electron-trapping property include transition metal complexes (e.g., an iridium complex and a platinum complex) whose ligands include a diazine skeleton such as a pyrimidine skeleton or a pyrazine skeleton. Note that the phosphorescent substance exhibits an electron-trapping property when the LUMO level of the phosphorescent substance is lower than the LUMO levels of both of the first organic compound and the second organic compound.

Note that in the light-emitting element, light emitted from the first light-emitting layer 113 a preferably has a peak on the shorter wavelength side than light emitted from the second light-emitting layer 113 b. The luminance of a light-emitting element using the phosphorescent substance emitting light with a short wavelength tends to degrade quickly. In view of the above, fluorescence with a short wavelength is used, so that a light-emitting element with less degradation of luminance can be provided.

The number and thicknesses of layers forming the EL layer are smaller in the light-emitting element of one embodiment of the present invention than in a tandem element; thus, the light-emitting element of one embodiment of the present invention is cost-effective and suitable for mass production. In addition, the number of layers forming the EL layer is small as described above; thus, the thickness of the EL layer can be small and the light-emitting element is optically advantageous (i.e., the outcoupling efficiency is high). Furthermore, the light-emitting element can have low drive voltage and provide both fluorescence and phosphorescence efficiently at a drive voltage of 5 V or lower.

Moreover, although the fluorescent layer and the phosphorescent layer are in contact with each other, deactivation of the triplet excitation energy is less likely to occur owing to the use of the above-described exciplex; thus, both phosphorescence and fluorescence can be obtained easily.

FIG. 21 shows a correlation between energy levels of substances and exciplexes in the light-emitting element described in this embodiment. In FIG. 21, S_(FH) denotes the singlet excitation level of the host material in the first light-emitting layer 113 a; T_(FH), the triplet excitation level of the host material in the first light-emitting layer 113 a; S_(FG) and T_(FG), the singlet excitation level and the triplet excitation level of a guest material (the fluorescent substance) in the first light-emitting layer 113 a, respectively; S_(PH) and T_(PH), the singlet excitation level and the triplet excitation level of a host material (the first organic compound or the second organic compound) in the second light-emitting layer 113 b, respectively; S_(E) and T_(E), the singlet excitation level and the triplet excitation level of the exciplex in the second light-emitting layer 113 b, respectively; and T_(PG), the triplet excitation level of a guest material (the phosphorescent substance) in the second light-emitting layer 113 b.

As shown in FIG. 21, TTA occurs because of collision of triplet excited molecules of the host materials in the first light-emitting layer 113 a and some of the triplet excited molecules of the host material are converted into singlet excited molecules while some of the triplet excited molecules are thermally decayed. Then, the singlet excitation energy of the host materials that is generated by TTA is transferred to the singlet excited state of the fluorescent substance, and the singlet excitation energy is converted into fluorescence.

In the second light-emitting layer 113 b, the excitation levels S_(E) and T_(E) of the exciplex are lower than the excitation levels S_(PH) and T_(PH) of the host materials (the first organic compound and the second organic compound); thus, excitation energy transfer from the exciplex to the host material does not occur. In addition, needless to say, energy transfer from the exciplex to another exciplex does not occur. When the excitation energy of the exciplex is transferred to the guest material (the phosphorescent substance), the excitation energy is converted into luminescence. As described above, the triplet excitation energy is hardly diffused and is converted into luminescence in the second light-emitting layer 113 b.

Since the triplet excitation energy is hardly diffused, light emission can be obtained with high efficiency from both of the first light-emitting layer 113 a and the second light-emitting layer 113 b in spite of a little energy transfer at the interface between the first light-emitting layer 113 a and the second light-emitting layer 113 b (e.g., energy transfer from T_(PG) of the phosphorescent substance at the interface to T_(FH) or T_(FG)). Note that in the first light-emitting layer 113 a, the singlet excited state is generated by the triplet excitation energy in TTA, and part of the energy transfer at the interface is converted into fluorescence. This can suppress loss of the energy.

In the light-emitting element of this embodiment, the first light-emitting layer 113 a and the second light-emitting layer 113 b are made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. The emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks.

Such a light-emitting element is suitable for obtaining white light emission. When the first light-emitting layer 113 a and the second light-emitting layer 113 b emit light of complementary colors, white light emission can be obtained. In addition, white light emission with a high color rendering property that is formed of three colors or four or more colors can be obtained by using a plurality of light-emitting substances emitting light with different wavelengths for one or both of the light-emitting layers. In that case, each of the light-emitting layers may be divided into layers and the divided layers may contain different light-emitting substances.

The lowest-energy-side absorption band of the phosphorescent substance overlaps the emission spectrum of the exciplex in the second light-emitting layer 113 b, whereby the light-emitting element can have higher emission efficiency. The difference in equivalent energy value between a peak wavelength in the lowest-energy-side absorption band of the phosphorescent substance and a peak wavelength of the emission spectrum of the exciplex is preferably less than or equal to 0.2 eV, in which case the overlap between the absorption band and the emission spectrum is large. Although the lowest-energy-side absorption band of the phosphorescent substance is preferably an absorption band of the triplet excitation level, the lowest-energy-side absorption band is preferably an absorption band of the singlet excitation level in the case where a TADF material is used instead of the phosphorescent substance as described later.

In FIG. 1A, the first light-emitting layer 113 a is formed on the side where the first electrode 101 functioning as the anode is formed and the second light-emitting layer 113 b is formed on the side where the second electrode 102 functioning as the cathode is formed. However, the stacking order may be reversed. In other words, the first light-emitting layer 113 a may be formed on the side where the second electrode 102 functioning as the cathode is formed and the second light-emitting layer 113 b may be formed on the side where the first electrode 101 functioning as the anode is formed.

Note that the structure of the light-emitting element in this embodiment is effective as long as the light-emitting substance contained in the second light-emitting layer 113 b can convert triplet excitation energy into luminescence. In the following description, a “phosphorescent substance” can be replaced by a “thermally activated delayed fluorescence (TADF) material”, and a “phosphorescent layer” can be replaced by a “TADF layer”. The TADF material is a substance that can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibits light emission (fluorescence) from the singlet excited state. The TADF is efficiently obtained under the condition where the difference in energy between the triplet excitation level and the singlet excitation level is greater than or equal to 0 eV and less than or equal to 0.2 eV, preferably greater than or equal to 0 eV and less than or equal to 0.1 eV. The phosphorescent substance and the TADF material are both substances that can convert triplet excitation energy into luminescence.

Embodiment 2

In this embodiment, a detailed example of the structure of the light-emitting element described in Embodiment 1 is described below with reference to FIGS. 1A and 1B.

A light-emitting element in this embodiment includes, between a pair of electrodes, an EL layer including a plurality of layers. In this embodiment, the light-emitting element includes the first electrode 101, the second electrode 102, and the EL layer 103 provided between the first electrode 101 and the second electrode 102. Note that in this embodiment, the first electrode 101 functions as an anode and the second electrode 102 functions as a cathode. In other words, when voltage is applied between the first electrode 101 and the second electrode 102 so that the potential of the first electrode 101 is higher than that of the second electrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the first electrode 101 is preferably formed using any of metals, alloys, electrically conductive compounds with a high work function (specifically, a work function of 4.0 eV or more), mixtures thereof, and the like. Specific examples include indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide, and indium oxide containing tungsten oxide and zinc oxide (IWZO). Films of these electrically conductive metal oxides are usually formed by a sputtering method but may be formed by application of a sol-gel method or the like. For example, indium oxide-zinc oxide is deposited by a sputtering method using a target obtained by adding 1 wt % to 20 wt % of zinc oxide to indium oxide. A film of indium oxide containing tungsten oxide and zinc oxide (IWZO) can be formed by a sputtering method using a target in which tungsten oxide and zinc oxide are added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitrides of metal materials (e.g., titanium nitride), and the like can be given. Graphene can also be used. Note that when a composite material described later is used for a layer that is in contact with the first electrode 101 in the EL layer 103, an electrode material can be selected regardless of its work function.

There is no particular limitation on the stacked-layer structure of the EL layer 103 as long as the light-emitting layer 113 has the structure described in Embodiment 1. For example, the EL layer 103 can be formed by combining a hole-injection layer, a hole-transport layer, the light-emitting layer, an electron-transport layer, an electron-injection layer, a carrier-blocking layer, an intermediate layer, and the like as appropriate. In this embodiment, the EL layer 103 has a structure in which the hole-injection layer 111, the hole-transport layer 112, the light-emitting layer 113, the electron-transport layer 114, and the electron-injection layer 115 are stacked in this order over the first electrode 101. Specific examples of materials used for each layer are given below.

The hole-injection layer 111 is a layer containing a substance having a high hole-injection property. Molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be used. Alternatively, the hole-injection layer 111 can be formed using a phthalocyanine-based compound such as phthalocyanine (abbreviation: H₂Pc) or copper phthalocyanine (abbreviation: CuPc); an aromatic amine compound such as 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB) or N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD); a high molecule compound such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS); or the like.

Alternatively, a composite material in which a substance having a hole-transport property contains a substance having an acceptor property can be used for the hole-injection layer 111. Note that the use of such a substance having a hole-transport property which contains a substance having an acceptor property enables selection of a material used to form an electrode regardless of its work function. In other words, besides a material having a high work function, a material having a low work function can be used for the first electrode 101. As the acceptor substance, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F₄-TCNQ), chloranil, and the like can be given. In addition, transition metal oxides can be given. Moreover, oxides of metals belonging to Groups 4 to 8 of the periodic table can be used. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron-accepting properties. Among these, molybdenum oxide is especially preferable because it is stable in the air, has a low hygroscopic property, and is easily handled.

As the substance having a hole-transport property used for the composite material, any of a variety of organic compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and high molecular compounds (e.g., oligomers, dendrimers, or polymers) can be used. Note that the organic compound used for the composite material is preferably an organic compound having a high hole-transport property. Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs or higher is preferably used. Specific examples of the organic compound that can be used as a substance having a hole-transport property in the composite material are given below.

Examples of the aromatic amine compounds are N,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation: DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD), and 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B).

Specific examples of the carbazole derivatives that can be used for the composite material are 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1).

Other examples of the carbazole derivatives that can be used for the composite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Examples of the aromatic hydrocarbon that can be used for the composite material are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene. Other examples are pentacene and coronene. The aromatic hydrocarbon that has a hole mobility of 1×10⁻⁶ cm²/Vs or higher and has 14 to 42 carbon atoms is particularly preferable.

Note that the aromatic hydrocarbons that can be used for the composite material may have a vinyl skeleton. Examples of the aromatic hydrocarbon having a vinyl group include 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi) and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

Other examples are high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: poly-TPD).

By providing a hole-injection layer, a high hole-injection property can be achieved to allow a light-emitting element to be driven at a low voltage.

The hole-transport layer 112 is a layer containing a substance having a hole-transport property. Examples of the substance having a hole-transport property are aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), and 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP). The substances listed here have high hole-transport properties and are mainly ones that have a hole mobility of 10⁻⁶ cm²/Vs or higher. An organic compound given as an example of the substance having a hole-transport property in the composite material described above can also be used for the hole-transport layer 112. Moreover, a high molecular compound such as poly(N-vinylcarbazole) (abbreviation: PVK) and poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also be used. Note that the layer that contains a substance having a hole-transport property is not limited to a single layer, and may be a stack of two or more layers including any of the above substances.

In the case where the first light-emitting layer 113 a is provided on the anode side in the light-emitting element of one embodiment of the present invention, the HOMO level of a substance used for the hole-transport layer 112 and the HOMO level of a host material in the first light-emitting layer 113 a are preferably close to each other (an energy difference of 0.2 eV or less). This can prevent capture of too many holes by trap states and enables holes to flow into the first light-emitting layer 113 a and the second light-emitting layer 113 b. Thus, fluorescence and phosphorescence can be easily obtained in a balanced manner with high efficiency.

The light-emitting layer 113 has the structure of the light-emitting layer 113 that is described in Embodiment 1. In other words, the first light-emitting layer 113 a and the second light-emitting layer 113 b are stacked in this order over the first electrode. A host material and a fluorescent substance are contained in the first light-emitting layer 113 a. A first organic compound, a second organic compound, and a substance that can convert triplet excitation energy into luminescence (a phosphorescent compound or a TADF material) are contained in the second light-emitting layer 113 b. In the light-emitting element of this embodiment, a combination of the first organic compound and the second organic compound forms an exciplex. The exciplex can provide energy for the substance that can convert triplet excitation energy into luminescence, so that light can be efficiently emitted from both of the first light-emitting layer 113 a and the second light-emitting layer 113 b.

Examples of a material that can be used as the fluorescent substance in the first light-emitting layer 113 a are given below. Fluorescent materials other than those given below can also be used.

Examples of the fluorescent substance are 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation: PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine (abbreviation: PAPP2BPy), N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), 4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine (abbreviation: 2YGAPPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene (abbreviation: TBP), 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), 9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), 2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1), 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD), 2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), 2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile (abbreviation: BisDCM), and 2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM). Condensed aromatic diamine compounds typified by pyrenediamine compounds such as 1,6FLPAPrn and 1,6mMemFLPAPrn are particularly preferable because of their high hole-trapping properties, high emission efficiency, and high reliability.

Examples of a substance that can be used as the host material in the first light-emitting layer 113 a are given below.

The examples include anthracene compounds such as 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA), 6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan (abbreviation: 2mBnfPPA), and 9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene (abbreviation: FLPPA). The use of a substance having an anthracene skeleton as the host material enables a light-emitting layer that has high emission efficiency and durability to be provided. In particular, CzPA, cgDBCzPA, 2mBnfPPA, and PCzPA are preferable because of their excellent characteristics.

A phosphorescent substance and a TADF material can be used as the substance that can convert triplet excitation energy into luminescence in the second light-emitting layer 113 b. Examples of the phosphorescent substance and the TADF material are given below.

Examples of the phosphorescent substance are an organometallic iridium complex having a 4H-triazole skeleton, such as tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-kN2]phenyl-kC}iridium(III) (abbreviation: Ir(mpptz-dmp)₃), tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Mptz)₃), or tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex having a 1H-triazole skeleton, such as tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(Mptz1-mp)₃) or tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex having an imidazole skeleton, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi)₃), or tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complex in which a phenylpyridine derivative having an electron-withdrawing group is a ligand, such as bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate (abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III) picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), or bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) acetylacetonate (abbreviation: FIr(acac)). These are compounds emitting blue phosphorescence and have an emission peak at 440 nm to 520 nm.

Other examples are organometallic iridium complexes having pyrimidine skeletons, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)₃), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)₂(acac)), (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)₂(acac)), (acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(nbppm)₂(acac)), (acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(mpmppm)₂(acac)), and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes having pyrazine skeletons, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)₂(acac)) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexes having pyridine skeletons, such as tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃), bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate (abbreviation: Ir(ppy)₂acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq)₂(acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃), tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃), and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate (abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such as tris(acetylacetonato) (monophenanthroline)terbium(III) (abbreviation: Tb(acac)₃(Phen)). These are mainly compounds emitting green phosphorescence and have an emission peak at 500 nm to 600 nm. Note that an organometallic iridium complex having a pyrimidine skeleton has distinctively high reliability and emission efficiency and is thus especially preferable.

Other examples are (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) (abbreviation: Ir(5mdppm)₂(dibm)), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(5mdppm)₂(dpm)), and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(d1npm)₂(dpm)); organometallic iridium complexes having pyrazine skeletons, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), or (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes having pyridine skeletons, such as tris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(piq)₃) and bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP); and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)). These are compounds emitting red phosphorescence and have an emission peak at 600 nm to 700 nm. The organometallic iridium complex having a pyrazine skeleton can provide red light emission with favorable chromaticity.

Phosphorescent materials other than those given above may be used.

Materials given below can be used as the TADF material.

A fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, or the like can be used. A metal-containing porphyrin such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd) can be used. Examples of the metal-containing porphyrin are a protoporphyrin-tin fluoride complex (SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF₂(OEP)), an etioporphyrin-tin fluoride complex (SnF₂(Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which are represented by Structural Formulae below.

Alternatively, a heterocyclic compound including a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring can be used, such as 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), which is represented by Structural Formula below. The heterocyclic compound is preferably used because of the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferably used because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the energy difference between the S₁ level and the T₁ level becomes small.

There is no particular limitation on the materials that can be used as the first organic compound and the second organic compound as long as the combination of the materials satisfies the conditions described in Embodiment 1. A variety of carrier-transport materials can be selected.

Examples of the material having an electron-transport property are a heterocyclic compound having a polyazole skeleton, such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a heterocyclic compound having a polyazole skeleton such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), or 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II); a heterocyclic compound having a diazine skeleton, such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), or 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeleton such as 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoline (abbreviation: 2mDBTBPDBQu-II), 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among the above materials, a heterocyclic compound having a diazine skeleton and a heterocyclic compound having a pyridine skeleton have high reliability and are thus preferable. Specifically, a heterocyclic compound having a diazine (pyrimidine or pyrazine) skeleton has a high electron-transport property to contribute to a reduction in drive voltage.

Examples of the material having a hole-transport property are a compound having an aromatic amine skeleton, such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF), or N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine (abbreviation: PCBASF); a compound having a carbazole skeleton, such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound having a thiophene skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III), or 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV); and a compound having a furan skeleton, such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II). Among the above materials, a compound having an aromatic amine skeleton and a compound having a carbazole skeleton are preferable because these compounds are highly reliable and have high hole-transport properties to contribute to a reduction in drive voltage.

Carrier-transport materials can be selected from a variety of substances as well as from the carrier-transport materials given above. Note that as the first organic compound and the second organic compound, substances having a triplet level (a difference in energy between a ground state and a triplet excited state) higher than the triplet level of the phosphorescent compound are preferably selected. In addition, it is preferable that the combination of the first organic compound and the second organic compound be selected so that an exciplex which exhibits light emission whose wavelength overlaps a wavelength of a lowest-energy-side absorption band of the phosphorescent substance is formed.

Furthermore, the combination of a material having an electron-transport property as one of the first organic compound and the second organic compound and a material having a hole-transport property as the other organic compound is advantageous for the formation of an exciplex. The transport property of the light-emitting layer can be easily adjusted and a recombination region can be easily adjusted by changing the contained amount of each compound. The ratio of the contained amount of the material having a hole-transport property to the contained amount of the material having an electron-transport property may be 1:9 to 9:1.

The light-emitting layer 113 having the above-described structure can be formed by co-evaporation by a vacuum evaporation method, or an inkjet method, a spin coating method, a dip coating method, or the like using a mixed solution.

Note that although the structure in which the first light-emitting layer 113 a is formed on the anode side and the second light-emitting layer 113 b is formed on the cathode side is described in this embodiment, the stacking order may be reversed. In other words, the second light-emitting layer 113 b may be formed on the anode side and the first light-emitting layer 113 a may be formed on the cathode side.

The second light-emitting layer 113 b may be divided into two or more layers, and the divided layers may contain different light-emitting substances. In particular, a structure in which the second light-emitting layer 113 b is divided into a first phosphorescent layer that emits red light (i.e., light having an emission spectrum peak at 580 nm to 680 nm) and a second phosphorescent layer that emits green light (i.e., light having an emission spectrum peak at 500 nm to 560 nm) and the first light-emitting layer 113 a emits blue light (i.e., light having an emission spectrum peak at 400 nm to 480 nm) is preferably employed, in which case white light emission with a favorable color rendering property can be obtained. Note that in that case, the first light-emitting layer 113 a, the first phosphorescent layer, and the second phosphorescent layer are preferably stacked in this order for high durability. Furthermore the first light-emitting layer 113 a is preferably formed on the anode side, in which case favorable characteristics can be obtained.

The other structure and effect of the light-emitting layer 113 are the same as those described in Embodiment 1. Embodiment 1 is to be referred to.

The electron-transport layer 114 is a layer containing a substance having an electron-transport property. For example, the electron-transport layer 114 is formed using a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), or bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq), or the like. A metal complex having an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), or the like can also be used. Besides the metal complexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or the like can also be used. The substances listed here have high electron-transport properties and are mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or higher. Note that any of the host materials having electron-transport properties, which are listed above, may be used for the electron-transport layer 114.

The electron-transport layer 114 is not limited to a single layer, and may be a stack of two or more layers each containing any of the substances listed above.

A layer for controlling transport of electron carriers may be provided between the electron-transport layer and the light-emitting layer. This is a layer formed by addition of a small amount of a substance having a high electron-trapping property to the aforementioned materials having a high electron-transport property, and the layer is capable of adjusting carrier balance by retarding transport of electron carriers. Such a structure is very effective in preventing a problem (e.g., a reduction in element lifetime) caused when electrons pass through the light-emitting layer.

An electron-injection layer 115 may be provided in contact with the second electrode 102 between the electron-transport layer 114 and the second electrode 102. For the electron-injection layer 115, an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride (CaF₂), can be used. For example, a layer that is formed using a substance having an electron-transport property and contains an alkali metal, an alkaline earth metal, or a compound thereof can be used. Note that a layer that is formed using a substance having an electron-transport property and contains an alkali metal or an alkaline earth metal is preferably used as the electron-injection layer 115, in which case electron injection from the second electrode 102 is efficiently performed.

For the second electrode 102, any of metals, alloys, electrically conductive compounds, and mixtures thereof which have a low work function (specifically, a work function of 3.8 eV or less) or the like can be used. Specific examples of such a cathode material are elements belonging to Groups 1 and 2 of the periodic table, such as alkali metals (e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), and strontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metals such as europium (Eu) and ytterbium (Yb), and alloys thereof. However, when the electron-injection layer is provided between the second electrode 102 and the electron-transport layer, for the second electrode 102, any of a variety of conductive materials such as Al, Ag, ITO, or indium oxide-tin oxide containing silicon or silicon oxide can be used regardless of the work function. These conductive materials can be deposited by a sputtering method, an ink-jet method, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103 regardless whether it is a dry process or a wet process. For example, a vacuum evaporation method, an ink-jet method, or a spin coating method may be employed. A different formation method may be employed for each electrode or each layer.

The electrode may be formed by a wet method using a sol-gel method, or by a wet method using paste of a metal material. Alternatively, the electrode may be formed by a dry method such as a sputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure, current flows because of a potential difference between the first electrode 101 and the second electrode 102, and holes and electrons recombine in the light-emitting layer 113 that contains a substance with a high light-emitting property, so that light is emitted. That is, a light-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the first electrode 101 and the second electrode 102. Therefore, one or both of the first electrode 101 and the second electrode 102 are light-transmitting electrodes.

Note that the structure of EL layer 103 provided between the first electrode 101 and the second electrode 102 is not limited to the above-described structure. Preferably, a light-emitting region where holes and electrons recombine is positioned away from the first electrode 101 and the second electrode 102 so that quenching due to the proximity of the light-emitting region and a metal used for electrodes and carrier-injection layers can be prevented.

In order that transfer of energy from an exciton generated in the light-emitting layer can be suppressed, preferably, the hole-transport layer and the electron-transport layer that are in contact with the light-emitting layer 113, particularly a carrier-transport layer in contact with a side closer to the light-emitting region in the light-emitting layer 113, are formed using a substance having a wider band gap than the light-emitting substance of the light-emitting layer or the emission center substance included in the light-emitting layer.

A light-emitting element in this embodiment is preferably fabricated over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from the first electrode 101 side or sequentially stacked from the second electrode 102 side. In a light-emitting device, although one light-emitting element may be fabricated over one substrate, a plurality of light-emitting elements may be fabricated over one substrate. With a plurality of light-emitting elements as described above formed over one substrate, a lighting device in which elements are separated or a passive-matrix light-emitting device can be fabricated. A light-emitting element may be formed over an electrode electrically connected to an field-effect transistor (FET), for example, that is formed over a substrate of glass, plastic, or the like, so that an active matrix light-emitting device in which the FET controls the drive of the light-emitting element can be fabricated. Note that the structure of the FET is not particularly limited. In addition, crystallinity of a semiconductor used for the FET is not particularly limited either; an amorphous semiconductor or a crystalline semiconductor may be used. In addition, a driver circuit formed in an FET substrate may be formed with an n-type FET and a p-type FET, or with either an n-type FET or a p-type FET.

Note that this embodiment can be combined with any of the other embodiments as appropriate.

Next, one mode a light-emitting element with a structure in which a plurality of light-emitting units are stacked (hereinafter also referred to as a stacked-type element) is described with reference to FIG. 1B. In this light-emitting element, a plurality of light-emitting units are provided between a first electrode and a second electrode. One light-emitting unit has a structure similar to that of the EL layer 103, which is illustrated in FIG. 1A. In other words, the light-emitting element illustrated in FIG. 1A includes a single light-emitting unit; the light-emitting element in this embodiment includes a plurality of light-emitting units.

In FIG. 1B, a first light-emitting unit 511 and a second light-emitting unit 512 are stacked between a first electrode 501 and a second electrode 502, and a charge-generation layer 513 is provided between the first light-emitting unit 511 and the second light-emitting unit 512. The first electrode 501 and the second electrode 502 correspond, respectively, to the first electrode 101 and the second electrode 102 illustrated in FIG. 1A, and the materials given in the description for FIG. 1A can be used. Furthermore, the first light-emitting unit 511 and the second light-emitting unit 512 may have the same structure or different structures.

The charge-generation layer 513 contains a composite material of an organic compound and a metal oxide. As the composite material of the organic compound and the metal oxide, the composite material which can be used for the hole-injection layer 111 illustrated in FIG. 1A can be used. A compound having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferably used as the organic compound. Note that any other substance may be used as long as the substance has a hole-transport property higher than an electron-transport property. The composite material of the organic compound and the metal oxide can achieve low-voltage driving and low-current driving because of the excellent carrier-injection property and carrier-transport property. Note that when a surface of a light-emitting unit on the anode side is in contact with a charge generation layer, the charge generation layer can also serve as a hole-transport layer of the light-emitting unit; thus, a hole-transport layer does not need to be formed in the light-emitting unit.

The charge generation layer 513 may have a stacked-layer structure of a layer containing the composite material of an organic compound and a metal oxide and a layer containing another material. For example, a layer containing a composite material of the organic compound and the metal oxide may be combined with a layer containing a compound of a substance selected from substances with an electron-donating property and a compound with a high electron-transport property. Moreover, a layer containing a composite material of the organic compound and the metal oxide may be combined with a transparent conductive film.

The charge generation layer 513 interposed between the first light-emitting unit 511 and the second light-emitting unit 512 may have any structure as long as electrons can be injected to a light-emitting unit on one side and holes can be injected to a light-emitting unit on the other side when a voltage is applied between the first electrode 501 and the second electrode 502. For example, in FIG. 1B, any layer can be used as the charge generation layer 513 as long as the layer injects electrons into the first light-emitting unit 511 and holes into the second light-emitting unit 512 when a voltage is applied such that the potential of the first electrode is higher than that of the second electrode.

The light-emitting element having two light-emitting units is described with reference to FIG. 1B; however, the present invention can be similarly applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge generation layer between a pair of electrodes as in the light-emitting element of this embodiment, it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. Moreover, it is possible to achieve a light-emitting device which can be driven at low voltage and has low power consumption.

When the above-described structure of the light-emitting layer 113 is applied to at least one of the plurality of units, the number of manufacturing steps of the unit can be reduced; thus, a multicolor light-emitting element which is advantageous for practical application can be provided.

The above-described structure can be combined with any of the structures in this embodiment and the other embodiments.

Embodiment 3

In this embodiment, a light-emitting device including the light-emitting element described in Embodiment 1 or 2 is described.

In this embodiment, the light-emitting device fabricated using the light-emitting element described in Embodiment 1 or 2 is described with reference to FIGS. 2A and 2B. Note that FIG. 2A is a top view illustrating the light-emitting device and FIG. 2B is a cross-sectional view taken along lines A-B and C-D in FIG. 2A. This light-emitting device includes a driver circuit portion (source line driver circuit) 601, a pixel portion 602, and a driver circuit portion (gate line driver circuit) 603, which control light emission of the light-emitting element and denoted by dotted lines. A reference numeral 604 denotes a sealing substrate; 605, a sealant; and 607, a space surrounded by the sealant 605.

Note that a lead wiring 608 is a wiring for transmitting signals to be input to the source line driver circuit 601 and the gate line driver circuit 603 and for receiving a video signal, a clock signal, a start signal, a reset signal, and the like from an FPC (flexible printed circuit) 609 serving as an external input terminal. Although only the FPC is illustrated here, a printed wiring board (PWB) may be attached to the FPC. The light-emitting device in the present specification includes, in its category, not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG. 2B. The driver circuit portion and the pixel portion are formed over an element substrate 610. The source line driver circuit 601, which is a driver circuit portion, and one of the pixels in the pixel portion 602 are illustrated here.

In the source line driver circuit 601, a CMOS circuit is formed in which an n-channel FET 623 and a p-channel FET 624 are combined. In addition, the driver circuit may be formed with any of a variety of circuits such as a CMOS circuit, a PMOS circuit, and an NMOS circuit. Although a driver-integrated type in which a driver circuit is formed over a substrate is described in this embodiment, one embodiment of the present invention is not limited to this type, and the driver circuit can be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels including a switching FET 611, a current controlling FET 612, and a first electrode 613 electrically connected to a drain of the current controlling FET 612. An insulator 614 is formed to cover end portions of the first electrode 613. In this embodiment, the insulator 614 is formed using a positive photosensitive acrylic resin film.

The insulator 614 preferably has a curved surface with curvature at an upper end portion or a lower end portion thereof. For example, in the case where a positive photosensitive acrylic resin is used for a material of the insulator 614, only the upper end portion of the insulator 614 has a surface with a curvature radius (0.2 μm to 3 μm). As the insulator 614, either a negative photosensitive resin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the first electrode 613. As a material used for the first electrode 613 functioning as an anode, a material having a high work function is preferably used. For example, a single-layer film of an ITO film, an indium tin oxide film containing silicon, an indium oxide film containing zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, a chromium film, a tungsten film, a Zn film, a Pt film, or the like, a stack including a titanium nitride film and a film containing aluminum as its main component, a stack including three layers of a titanium nitride film, a film containing aluminum as its main component, and a titanium nitride film, or the like can be used. The stacked-layer structure enables low wiring resistance, favorable ohmic contact, and a function as an anode.

The EL layer 616 is formed by any of a variety of methods such as an evaporation method using an evaporation mask, an inkjet method, and a spin coating method. The EL layer 616 has a structure similar to that described in Embodiment 1 or 2. As another material contained in the EL layer 616, any of low-molecular-weight compounds and high-molecular-weight compounds (including oligomers and dendrimers) may be used.

As a material used for the second electrode 617, which is formed over the EL layer 616 and functions as a cathode, a material having a low work function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof, such as MgAg, MgIn, or AlLi) is preferably used. In the case where light generated in the EL layer 616 passes through the second electrode 617, a stack of a thin metal film and a transparent conductive film (e.g., ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt %, indium tin oxide containing silicon, or zinc oxide (ZnO)) is preferably used for the second electrode 617.

Note that the light-emitting element is formed with the first electrode 613, the EL layer 616, and the second electrode 617. The light-emitting element has the structure described in Embodiment 1 or 2. In the light-emitting device of this embodiment, the pixel portion, which includes a plurality of light-emitting elements, may include both of the light-emitting element described in Embodiment 1 or 2 and a light-emitting element having a different structure.

The sealing substrate 604 is attached to the element substrate 610 with the sealant 605, so that the light-emitting element 618 is provided in the space 607 surrounded by the element substrate 610, the sealing substrate 604, and the sealant 605. The space 607 may be filled with filler, and may be filled with an inert gas (such as nitrogen or argon), or the sealant 605. It is preferable that the sealing substrate be provided with a recessed portion and the drying agent 625 be provided in the recessed portion, in which case deterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealant 605. It is preferable that such a material not transmit moisture or oxygen as much as possible. As the sealing substrate 604, a glass substrate, a quartz substrate, or a plastic substrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used.

As described above, the light-emitting device that includes the light-emitting element described in Embodiment 1 or 2 can be obtained.

FIGS. 3A and 3B each illustrate an example of a light-emitting device in which full color display is achieved by forming a light-emitting element exhibiting white light emission and providing a coloring layer (a color filter) and the like. In FIG. 3A, a substrate 1001, a base insulating film 1002, a gate insulating film 1003, gate electrodes 1006, 1007, and 1008, a first interlayer insulating film 1020, a second interlayer insulating film 1021, a peripheral portion 1042, a pixel portion 1040, a driver circuit portion 1041, first electrodes 1024W, 1024R, 1024G, and 1024B of light-emitting elements, a partition wall 1025, an EL layer 1028, a second electrode 1029 of the light-emitting elements, a sealing substrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 3A, coloring layers (a red coloring layer 1034R, a green coloring layer 1034G, and a blue coloring layer 1034B) are provided on a transparent base material 1033. A black layer (a black matrix) 1035 may be additionally provided. The transparent base material 1033 provided with the coloring layers and the black layer is positioned and fixed to the substrate 1001. Note that the coloring layers and the black layer are covered with an overcoat layer 1036. In FIG. 3A, light emitted from part of the light-emitting layer does not pass through the coloring layers, while light emitted from the other part of the light-emitting layer passes through the coloring layers. Since light that does not pass through the coloring layers is white and light that passes through any one of the coloring layers is red, blue, or green, an image can be displayed using pixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) are provided between the gate insulating film 1003 and the first interlayer insulating film 1020. As shown in FIG. 3B, the coloring layers may be provided between the substrate 1001 and the sealing substrate 1031.

The above-described light-emitting device has a structure in which light is extracted from the substrate 1001 side where the FETs are formed (a bottom emission structure), but may have a structure in which light is extracted from the sealing substrate 1031 side (a top emission structure). FIG. 4 is a cross-sectional view of a light-emitting device having a top emission structure. In this case, a substrate that does not transmit light can be used as the substrate 1001. The process up to the step of forming of a connection electrode which connects the FET and the anode of the light-emitting element is performed in a manner similar to that of the light-emitting device having a bottom emission structure. Then, a third interlayer insulating film 1037 is formed to cover an electrode 1022. This insulating film may have a planarization function. The third interlayer insulating film 1037 can be formed using a material similar to that of the second interlayer insulating film, and can alternatively be formed using any other materials.

The first electrodes 1024W, 1024R, 1024G, and 1024B of the light-emitting elements each serve as an anode here, but may serve as a cathode. In the case of a light-emitting device having a top emission structure as illustrated in FIG. 4, the first electrodes are preferably reflective electrodes. The EL layer 1028 is formed to have a structure similar to the structure of the EL layer 103 described in Embodiment 1 or 2, with which white light emission can be obtained.

In the case of a top emission structure as illustrated in FIG. 4, sealing can be performed with the sealing substrate 1031 on which the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) are provided. The sealing substrate 1031 may be provided with the black layer (the black matrix) 1035 that is positioned between pixels. The coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) and the black layer (the black matrix) 1035 may be covered with an overcoat layer. Note that a light-transmitting substrate is used as the sealing substrate 1031.

Although an example in which full color display is performed using four colors of red, green, blue, and white is shown here, there is no particular limitation and full color display using three colors of red, green, and blue may be performed.

The light-emitting device in this embodiment is fabricated using the light-emitting element described in Embodiment 1 or 2 and thus can have favorable characteristics. Specifically, since the light-emitting element described in Embodiment 1 or 2 has high emission efficiency, the light-emitting device can have reduced power consumption. In addition, since the light-emitting element is easily mass-produced, the light-emitting device can be provided at low cost.

An active matrix light-emitting device is described above, whereas a passive matrix light-emitting device is described below. FIGS. 5A and 5B illustrate a passive matrix light-emitting device fabricated using the present invention. FIG. 5A is a perspective view of the light-emitting device, and FIG. 5B is a cross-sectional view taken along the line X-Y in FIG. 5A. In FIGS. 5A and 5B, an EL layer 955 is provided between an electrode 952 and an electrode 956 over a substrate 951. An end portion of the electrode 952 is covered with an insulating layer 953. In addition, a partition layer 954 is provided over the insulating layer 953. The sidewalls of the partition layer 954 slope so that the distance between one sidewall and the other sidewall gradually decreases toward the surface of the substrate. In other words, a cross section taken along the direction of the short side of the partition layer 954 is trapezoidal, and the base (a side which is in the same direction as a plane direction of the insulating layer 953 and in contact with the insulating layer 953) is shorter than the upper side (a side which is in the same direction as the plane direction of the insulating layer 953 and not in contact with the insulating layer 953). By providing the partition layer 954 in such a manner, a defect of the light-emitting element due to static electricity or the like can be prevented. The passive matrix light-emitting device also includes the light-emitting element described in Embodiment 1 or 2, which has high emission efficiency, and thus can have less power consumption. Moreover, since the light-emitting element is easily mass-produced, the light-emitting device can be provided at low cost.

Since many minute light-emitting elements arranged in a matrix in the light-emitting device described above can each be controlled, the light-emitting device can be suitably used as a display device for displaying images.

This embodiment can be freely combined with any of the other embodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting element described in Embodiment 1 or 2 is used for a lighting device is described with reference to FIGS. 6A and 6B. FIG. 6B is a top view of the lighting device, and FIG. 6A is a cross-sectional view taken along the line e-f in FIG. 6B.

In the lighting device in this embodiment, a first electrode 401 is formed over a substrate 400 which is a support and has a light-transmitting property. The first electrode 401 corresponds to the first electrode 101 in Embodiment 1. When light is extracted through the first electrode 401 side, the first electrode 401 is formed using a material having a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is provided over the substrate 400.

An EL layer 403 is formed over the first electrode 401. The structure of the EL layer 403 corresponds to, for example, the structure of the EL layer 103 in Embodiment 1, or the structure in which the light-emitting units 511 and 512 and the charge-generation layer 513 are combined. For these structures, the description in Embodiment 1 can be referred to.

The second electrode 404 is formed to cover the EL layer 403. The second electrode 404 corresponds to the second electrode 102 in Embodiment 1. The second electrode 404 is formed using a material having high reflectance when light is extracted through the first electrode 401 side. The second electrode 404 is connected to the pad 412, whereby voltage is applied thereto.

As described above, the lighting device described in this embodiment includes a light-emitting element including the first electrode 401, the EL layer 403, and the second electrode 404. Since the light-emitting element has high emission efficiency, the lighting device in this embodiment can have low power consumption.

The light-emitting element having the above structure is fixed to a sealing substrate 407 with sealants 405 and 406 and sealing is performed, whereby the lighting device is completed. It is possible to use only either the sealant 405 or the sealant 406. In addition, the inner sealant 406 (not illustrated in FIG. 6B) can be mixed with a desiccant that enables moisture to be adsorbed, which results in improved reliability.

When parts of the pad 412 and the first electrode 401 are extended to the outside of the sealants 405 and 406, the extended parts can serve as external input terminals. An IC chip 420 mounted with a converter or the like may be provided over the external input terminals.

As described above, since the lighting device described in this embodiment includes the light-emitting element described in Embodiment 1 or 2 as an EL element, the lighting device can have low power consumption. In addition, the light-emitting device can have low drive voltage. Furthermore, the light-emitting device can be inexpensive.

Embodiment 5

In this embodiment, examples of electronic appliances each including the light-emitting element described in Embodiment 1 or 2 are described. The light-emitting element described in Embodiment 1 or 2 has high emission efficiency and reduced power consumption. As a result, the electronic appliances described in this embodiment can each include a light-emitting portion having reduced power consumption. The light-emitting element described in Embodiment 1 or 2 includes a smaller number of layers to be formed; thus, the electronic appliances can be inexpensive.

Examples of the electronic appliance to which the above light-emitting element is applied include television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as cellular phones or cellular phone devices), portable game machines, portable information terminals, audio playback devices, and large game machines such as pachinko machines. Specific examples of these electronic appliances are given below.

FIG. 7A illustrates an example of a television device. In the television device, a display portion 7103 is incorporated in a housing 7101. In addition, here, the housing 7101 is supported by a stand 7105. Images can be displayed on the display portion 7103 where the light-emitting elements described in Embodiment 1 or 2 are arranged in a matrix.

The television device can be operated with an operation switch of the housing 7101 or a separate remote controller 7110. With operation keys 7109 of the remote controller 7110, channels and volume can be controlled and images displayed on the display portion 7103 can be controlled. Furthermore, the remote controller 7110 may be provided with a display portion 7107 for displaying data output from the remote controller 7110.

Note that the television device is provided with a receiver, a modem, and the like. With the receiver, general television broadcasts can be received. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) data communication can be performed.

FIG. 7B1 illustrates a computer that includes a main body 7201, a housing 7202, a display portion 7203, a keyboard 7204, an external connection port 7205, a pointing device 7206, and the like. Note that this computer is manufactured by using light-emitting elements arranged in a matrix in the display portion 7203, which are the same as that described in Embodiment 1 or 2. The computer illustrated in FIG. 7B1 may have a structure illustrated in FIG. 7B2. A computer illustrated in FIG. 7B2 is provided with a second display portion 7210 instead of the keyboard 7204 and the pointing device 7206. The second display portion 7210 is a touch screen, and input can be performed by operation of display for input on the second display portion 7210 with a finger or a dedicated pen. The second display portion 7210 can also display images other than the display for input. The display portion 7203 may also be a touchscreen. Connecting the two screens with a hinge can prevent troubles; for example, the screens can be prevented from being cracked or broken while the computer is being stored or carried. Note that this computer is manufactured by arranging the light-emitting elements described in Embodiment 1 or 2 in a matrix in the display portion 7203.

FIG. 7C illustrates a portable game machine that includes two housings, a housing 7301 and a housing 7302, which are connected with a joint portion 7303 so that the portable game machine can be opened or folded. The housing 7301 incorporates a display portion 7304 including the light-emitting elements described in Embodiment 1 or 2 and arranged in a matrix, and the housing 7302 incorporates a display portion 7305. In addition, the portable game machine illustrated in FIG. 7C includes a speaker portion 7306, a recording medium insertion portion 7307, an LED lamp 7308, an input means (an operation key 7309, a connection terminal 7310, a sensor 7311 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), or a microphone 7312), and the like. Needless to say, the structure of the portable game machine is not limited to the above structure as long as the display portion including the light-emitting elements each of which is described in Embodiment 1 or 2 and which are arranged in a matrix is used as at least either the display portion 7304 or the display portion 7305, or both, and the structure can include other accessories as appropriate. The portable game machine illustrated in FIG. 7C has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. The portable game machine illustrated in FIG. 7C can have a variety of functions without limitation to the above functions.

FIG. 7D illustrates an example of a mobile phone. The mobile phone is provided with a display portion 7402 incorporated in a housing 7401, an operation button 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone has the display portion 7402 including the light-emitting elements each of which is described in Embodiment 1 or 2 and which are arranged in a matrix.

When the display portion 7402 of the mobile phone illustrated in FIG. 7D is touched with a finger or the like, data can be input into the mobile phone. In this case, operations such as making a call and creating an e-mail can be performed by touch on the display portion 7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. The first mode is a display mode mainly for displaying images. The second mode is an input mode mainly for inputting data such as text. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are combined.

For example, in the case of making a call or composing an e-mail, a text input mode mainly for inputting text is selected for the display portion 7402 so that text displayed on a screen can be inputted. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion 7402.

When a detection device which includes a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone, the direction of the mobile phone (whether the mobile phone is placed horizontally or vertically for a landscape mode or a portrait mode) is determined so that display on the screen of the display portion 7402 can be automatically switched.

The screen modes are switched by touch on the display portion 7402 or operation with the operation button 7403 of the housing 7401. The screen modes can be switched depending on the kind of images displayed on the display portion 7402. For example, when a signal of an image displayed on the display portion is a signal of moving image data, the screen mode is switched to the display mode. When the signal is a signal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion 7402 is not performed for a certain period while a signal detected by an optical sensor in the display portion 7402 is detected, the screen mode may be controlled so as to be switched from the input mode to the display mode.

The display portion 7402 may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion 7402 with the palm or the finger, whereby personal authentication can be performed. In addition, by providing a backlight or a sensing light source that emits a near-infrared light in the display portion, an image of a finger vein, a palm vein, or the like can be taken.

Note that the structure described in this embodiment can be combined with any of the structures described in Embodiments 1 to 4 as appropriate.

As described above, the application range of the light-emitting device including the light-emitting element described in Embodiment 1 or 2 is so wide that the light-emitting device can be applied to electronic appliances in a variety of fields. By using the light-emitting element described in Embodiment 1 or 2, an electronic appliance having reduced power consumption can be obtained.

FIG. 8 illustrates an example of a liquid crystal display device using the light-emitting element described in Embodiment 1 or 2 for a backlight. The liquid crystal display device illustrated in FIG. 8 includes a housing 901, a liquid crystal layer 902, a backlight unit 903, and a housing 904. The liquid crystal layer 902 is connected to a driver IC 905. The light-emitting element described in Embodiment 1 or 2 is used for the backlight unit 903, to which current is supplied through a terminal 906.

The light-emitting element described in Embodiment 1 or 2 is used for the backlight of the liquid crystal display device; thus, the backlight can have reduced power consumption. In addition, the use of the light-emitting element described in Embodiment 2 enables fabrication of a planar-emission lighting device and further a larger-area planar-emission lighting device; therefore, the backlight can be a larger-area backlight, and the liquid crystal display device can also be a larger-area device. Furthermore, the light-emitting device using the light-emitting element described in Embodiment 2 can be thinner than a conventional one; accordingly, the display device can also be thinner.

FIG. 9 illustrates an example in which the light-emitting element described in Embodiment 1 or 2 is used for a table lamp that is a lighting device. The table lamp illustrated in FIG. 9 includes a housing 2001 and a light source 2002. The lighting device described in Embodiment 4 is used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting element described in Embodiment 1 or 2 is used for an indoor lighting device 3001. Since the light-emitting element described in Embodiment 1 or 2 has reduced power consumption, a lighting device having reduced power consumption can be obtained. In addition, since the light-emitting element described in Embodiment 1 or 2 can have a large area, the light-emitting element can be used for a large-area lighting device. Furthermore, since the light-emitting element described in Embodiment 1 or 2 is thin, the light-emitting element can be used for a lighting device having a reduced thickness.

The light-emitting element described in Embodiment 1 or 2 can also be used for an automobile windshield or an automobile dashboard. FIG. 11 illustrates one mode in which the light-emitting element described in Embodiment 2 is used for an automobile windshield and an automobile dashboard. Displays regions 5000 to 5005 each include the light-emitting element described in Embodiment 1 or 2.

The display region 5000 and the display region 5001 are provided in the automobile windshield in which the light-emitting elements described in Embodiment 1 or 2 are incorporated. The light-emitting element described in Embodiment 1 or 2 can be formed into what is called a see-through display device, through which the opposite side can be seen, by including a first electrode and a second electrode formed of electrodes having light-transmitting properties. Such a see-through display device does not hinder the vision and thus can be provided in the automobile windshield. Note that in the case where a transistor for driving or the like is provided, a transistor having a light-transmitting property, such as an organic transistor using an organic semiconductor material or a transistor using an oxide semiconductor, is preferably used.

A display region 5002 is provided in a pillar portion in which the light-emitting element described in Embodiment 1 or 2 are incorporated. The display region 5002 can compensate for the view hindered by the pillar portion by showing an image taken by an imaging unit provided in the car body. Similarly, the display region 5003 provided in the dashboard can compensate for the view hindered by the car body by showing an image taken by an imaging unit provided in the outside of the car body, which leads to elimination of blind areas and enhancement of safety. Showing an image so as to compensate for the area that a driver cannot see, makes it possible for the driver to confirm safety easily and comfortably.

The display region 5004 and the display region 5005 can provide a variety of kinds of information such as navigation data, a speedometer, a tachometer, a mileage, a fuel meter, a gearshift indicator, and air-condition setting. The contents or layout of the display can be changed by a user as appropriate. Note that such information can also be shown by the display regions 5000 to 5003. The display regions 5000 to 5005 can also be used as lighting devices.

The light-emitting element described in Embodiment 1 or 2 can have high emission efficiency and low power consumption. Therefore, load on a battery is small even when a number of large screens such as the display regions 5000 to 5005 are provided, which provides comfortable use. For that reason, the light-emitting device and the lighting device each of which includes the light-emitting element described in Embodiment 1 or 2 can be suitably used as an in-vehicle light-emitting device and an in-vehicle lighting device.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal. In FIG. 12A, the tablet terminal is opened and includes a housing 9630, a display portion 9631 a, a display portion 9631 b, a display-mode switching button 9034, a power button 9035, a power-saving-mode switching button 9036, a clip 9033, and an operation button 9038. Note that in the tablet terminal, one or both of the display portion 9631 a and the display portion 9631 b is/are formed using a light-emitting device which includes the light-emitting element described in Embodiment 1 or 2.

Part of the display portion 9631 a can be a touchscreen region 9632 a and data can be input when a displayed operation key 9637 is touched. Although a structure in which a half region in the display portion 9631 a has only a display function and the other half region also has a touchscreen function is illustrated as an example, the structure of the display portion 9631 a is not limited thereto. The whole area of the display portion 9631 a may have a touchscreen function. For example, the whole area of the display portion 9631 a can display keyboard buttons and serve as a touchscreen while the display portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b can be a touchscreen region 9632 b. When a switching button 9639 for showing/hiding a keyboard on the touchscreen is touched with a finger, a stylus, or the like, the keyboard can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touchscreen regions 9632 a and 9632 b.

The display-mode switching button 9034 allows switching between a portrait mode and a landscape mode, and between monochrome display and color display, for example. With the power-saving-mode switching button 9036, the luminance of display can be optimized in accordance with the amount of external light at the time when the tablet terminal is in use, which is detected with an optical sensor incorporated in the tablet terminal. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b have the same display area in FIG. 12A, one embodiment of the present invention is not limited to this example. The display portion 9631 a and the display portion 9631 b may have different areas or different display quality. For example, one of them may be a display panel that can display higher-definition images than the other.

In FIG. 12B, the tablet terminal is folded and includes the housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635, and a DC-to-DC converter 9636. Note that FIG. 12B illustrates an example in which the charge and discharge control circuit 9634 includes the battery 9635 and the DC-to-DC converter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closed when the tablet terminal is not used. Thus, the display portions 9631 a and 9631 b can be protected, whereby a tablet terminal with high endurance and high reliability for long-term use can be provided.

The tablet terminal illustrated in FIGS. 12A and 12B can also have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing data displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tablet terminal, supplies electric power to a touchscreen, a display portion, an image signal processor, and the like. Note that the solar cell 9633 is preferably provided on one or two surfaces of the housing 9630, in which case the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit 9634 illustrated in FIG. 12B are described with reference to a block diagram of FIG. 12C. FIG. 12C illustrates the solar cell 9633, the battery 9635, the DC-to-DC converter 9636, a converter 9638, switches SW1 to SW3, and the display portion 9631. The battery 9635, the DC-to-DC converter 9636, the converter 9638, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 in FIG. 12B.

First, an example of operation in the case where power is generated by the solar cell 9633 using external light is described. The voltage of power generated by the solar cell is raised or lowered by the DC-to-DC converter 9636 so that the power has voltage for charging the battery 9635. Then, when power supplied from the battery 9635 charged by the solar cell 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9638 so as to be voltage needed for the display portion 9631. In addition, when display on the display portion 9631 is not performed, the switch SW1 is turned off and a switch SW2 is turned on so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a power generation means, the power generation means is not particularly limited, and the battery 9635 may be charged by another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). The battery 9635 may be charged by a non-contact power transmission module that is capable of charging by transmitting and receiving power by wireless (without contact), or another charge means used in combination, and the power generation means is not necessarily provided.

One embodiment of the present invention is not limited to the tablet terminal having the shape illustrated in FIGS. 12A to 12C as long as the display portion 9631 is included.

Example 1

In this example, a light-emitting element (a light-emitting element 1) of one embodiment of the present invention is described. Note that the light-emitting element 1 included the light-emitting layer 113 including a fluorescent layer (the first light-emitting layer 113 a) and a phosphorescent layer (the second light-emitting layer 113 b) that were in contact with each other. The phosphorescent layer (the second light-emitting layer 113 b) was formed of a stack of a first phosphorescent layer (the second light-emitting layer 113 b)-1 emitting red phosphorescence and a second phosphorescent layer (the second light-emitting layer 113 b)-2 emitting green phosphorescence. Structural formulae of organic compounds used for the light-emitting element 1 are shown below.

A method for fabricating the light-emitting element 1 of this example is described below.

(Method for Fabricating Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate by a sputtering method to form the first electrode 101. The thickness was 110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode 101 functions as an anode of the light-emitting element.

Next, in pretreatment for forming the light-emitting element over the substrate, a surface of the substrate was washed with water and baked at 200° C. for 1 hour, and then UV ozone treatment was performed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10⁻⁴ Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to a substrate holder provided in the vacuum evaporation apparatus so that the surface on which the first electrode 101 was formed faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. After that, on the first electrode 101, 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxide were deposited by co-evaporation by an evaporation method using resistance heating to form the hole-injection layer 111. The thickness was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2 (=DBT3P-II: molybdenum oxide). Note that the co-evaporation method refers to an evaporation method in which evaporation is carried out from a plurality of evaporation sources at the same time in one treatment chamber.

Next, on the hole-injection layer 111, 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA) represented by Structural Formula (ii) was deposited to a thickness of 20 nm to form the hole-transport layer 112.

On the hole-transport layer 112, 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA) represented by Structural Formula (iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv) were deposited by co-evaporation to a thickness of 10 nm to form the fluorescent layer (the first light-emitting layer 113 a) so that the weight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.04 (=cgDBCzPA: 1,6mMemFLPAPrn). After that, 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula (vi), and (dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)]) represented by Structural Formula (vii) were deposited by co-evaporation to a thickness of 5 nm to form the first phosphorescent layer (the second light-emitting layer 113 b)-1 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tppr)₂(dpm)] was 0.6:0.4:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(tppr)₂(dpm)]), and then 2mDBTBPDBq-II, PCBBiF, and bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula (viii) were deposited by co-evaporation to a thickness of 20 nm to form the second phosphorescent layer (the second light-emitting layer 113 b)-2 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(tBuppm)₂(acac)]). Thus, the phosphorescent layer (the second light-emitting layer 113 b) was formed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in the phosphorescent layer (the second light-emitting layer 113 b). Specifically, the photoluminescence wavelength of a co-evaporation film of 2mDBTBPDBq-II and PCBBiF (i.e., the emission wavelength of the exciplex) is around 515 nm. This emission wavelength overlaps absorption bands on the longest wavelength sides of [Ir(tppr)₂(dpm)] and [Ir(tBuppm)₂(acac)], so that energy transfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in the fluorescent layer is higher than the singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescent substance. In addition, the triplet excitation energy of cgDBCzPA is lower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in the fluorescent layer (the first light-emitting layer 113 a), regeneration of a singlet exciton associated with triplet-triplet annihilation and light emission are obtained easily. Occurrence of delayed fluorescence was actually observed in the above-described structure.

After that, on the phosphorescent layer (the second light-emitting layer 113 b), 2mDBTBPDBq-II was deposited to a thickness of 10 nm, and bathophenanthroline (abbreviation: BPhen) represented by Structural Formula (ix) was deposited to a thickness of 15 nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102 functioning as a cathode. Through the above-described steps, the light-emitting element 1 of this example was fabricated.

Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method.

In a glove box under a nitrogen atmosphere, the light-emitting element 1 was sealed with a glass substrate so as not to be exposed to the air (specifically, a sealant was applied onto an outer edge of the element, and at the time of sealing, UV treatment was performed first and then heat treatment was performed at 80° C. for 1 hour). Then, the reliability of the light-emitting element 1 was measured. Note that the measurement was performed at room temperature (in the atmosphere kept at 25° C.).

FIG. 13 shows current density-luminance characteristics of the light-emitting element 1. FIG. 14 shows luminance-current efficiency characteristics of the light-emitting element 1. FIG. 15 shows voltage-luminance characteristics of the light-emitting element 1. FIG. 16 shows luminance-external quantum efficiency characteristics of the light-emitting element 1. FIG. 17 shows an emission spectrum of the light-emitting element 1.

As can be seen from the characteristics, although the light-emitting element 1 did not include an intermediate layer, the light-emitting element 1 had a current efficiency of approximately 30 cd/A at around 1000 cd/m² and an external quantum efficiency of approximately 13%. This indicates that the light-emitting element 1 had high emission efficiency. In addition, the drive voltage of the light-emitting element 1 was low, which was higher than or equal to 3 V and lower than 4 V.

Moreover, the emission spectrum shows that red light emission originating from [Ir(tppr)₂(dpm)], green light emission originating from [Ir(tBuppm)₂(acac)], and blue light emission originating from 1,6mMemFLPAPrn were observed. This indicates that light emission was sufficiently obtained from both of the fluorescent layer (the first light-emitting layer 113 a) and the phosphorescent layer (the second light-emitting layer 113 b). Furthermore, the light-emitting element 1 had a correlated color temperature of 3130 K at around 1000 cd/m² and a general color rendering index of 92, which means that the light-emitting element 1 had a sufficient color temperature for lighting and an excellent color rendering property.

FIG. 18 shows results of a reliability test performed on the light-emitting element 1. In the reliability test, the light-emitting element 1 was driven under conditions that the initial luminance was 5000 cd/m² and the current density was constant. FIG. 18 shows a change in normalized luminance with an initial luminance of 100%. The results show that the light-emitting element 1 kept 94% of the initial luminance even after being driven for 62 hours and that the light-emitting element 1 had a small decrease in luminance with driving time and high reliability.

The singlet excitation levels (S levels) of cgDBCzPA and 1,6mMemFLPArn that were used for the light-emitting element 1 were estimated to be 2.95 eV and 2.68 eV, respectively, from absorption edges of the co-evaporation film.

Table 1 shows measurement results of the triplet levels (T1 levels) of 2mDBTBPDBq-II, PCBBiF, cgDBCzPA, and 1,6mMemFLPAPrn that were used for the light-emitting element 1 in this example. The T1 levels were obtained by measurement of emission of phosphorescence from the substances. In the measurement, each substance was irradiated with excitation light with a wavelength of 325 nm and the measurement temperature was 10 K. In measurement of an energy level, calculation from an absorption wavelength is more accurate than calculation from an emission wavelength. However, absorption of the T1 level is extremely low and difficult to measure; thus, here, a peak wavelength located on the shortest wavelength side in a phosphorescence spectrum was regarded as the T1 level. For that reason, a few errors may be included in the measured values. Note that since intersystem crossing hardly occurs in cgDBCzPA and 1,6mMemFLPAPrn, tris(2-phenylpyridinato)iridium (abbreviation: Ir(ppy)₃) was added (i.e., co-evaporated) as a sensitizer, whereby phosphorescence was observed.

TABLE 1 Phosphorescene Peak Wavelength (nm) T1 level (eV) 2mDBTBPDBq-II 515 2.41 PCBBiF 509 2.44 cgDBCzPA 721 1.72 1,6mMemFLPAPrn 675 1.84

The above results indicate that in the fluorescent layer of the light-emitting element 1, the singlet excitation level of cgDBCzPA that was the host material was higher than that of 1,6mMemFLPAPrn that was the fluorescent substance and the triplet excitation level of cgDBCzPA was lower than that of 1,6mMemFLPAPrn; thus, the fluorescent layer (the first light-emitting layer 113 a) had a structure in which regeneration of a single exciton associated with triplet-triplet annihilation and light emission were obtained easily.

The results also indicate that the triplet excitation level of cgDBCzPA that was the host material in the fluorescent layer was lower than those of a first organic compound (2mDBTBPDBq-II) and a second organic compound (PCBBiF) in the phosphorescent layer. In the case of such a structure, many of triplet excitons generated in the phosphorescent layer are generally diffused into the fluorescent layer and non-radiative decay is caused. However, in the light-emitting element 1 of this example, the first organic compound and the second organic compound formed an exciplex; thus, triplet excitons generated in the phosphorescent layer were hardly diffused into the fluorescent layer. One reason is probably as follows: energy transfer from one exciplex to another exciplex is unlikely to occur because the exciplexes do not have ground states. Consequently, the light-emitting element 1 had remarkable characteristics: light emission from both of the fluorescent layer and the phosphorescent layer and high efficiency.

As described above, the light-emitting element 1 of one embodiment of the present invention had highly well-balanced, favorable characteristics and was able to be fabricated easily and inexpensively. The above-described results were attributed to the following: diffusion of excitons was suppressed and non-radiative decay of the triplet excitation energy was reduced by using the exciplex as an energy donor of the phosphorescent layer, and the emission efficiency was improved because of occurrence of delayed fluorescence due to triplet-triplet annihilation in the host material in the fluorescent layer.

Example 2

In this example, methods for fabricating a light-emitting element 2 and a light-emitting element 3 of embodiments of the present invention and characteristics thereof are described. Structural formulae of organic compounds used for the light-emitting element 2 and the light-emitting element 3 are shown below.

(Method for Fabricating Light-Emitting Element 2)

A film of indium tin oxide containing silicon oxide (JTSO) was formed to a thickness of 110 nm over a glass substrate by a sputtering method to form the first electrode 101. The electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over the substrate, a surface of the substrate was washed with water and baked at 200° C. for 1 hour, and then UV ozone treatment was performed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10⁻⁴ Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to a substrate holder provided in the vacuum evaporation apparatus so that the surface on which the first electrode 101 was formed faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. After that, on the first electrode 101, 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxide were deposited by co-evaporation by an evaporation method using resistance heating to form the hole-injection layer 111. The thickness was set to 30 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to 2:1 (=DBT3P-II: molybdenum oxide).

Next, on the hole-injection layer 111, 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA) represented by Structural Formula (ii) was deposited to a thickness of 20 nm to form the hole-transport layer 112.

On the hole-transport layer 112, 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA) represented by Structural Formula (iii) and N,N-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv) were deposited by co-evaporation to a thickness of 10 nm to form the first light-emitting layer 113 a that was a fluorescent layer so that the weight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.02 (=cgDBCzPA: 1,6mMemFLPAPrn). After that, 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula (vi), and bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(dmdppr-dmp)₂(acac)]) represented by Structural Formula (x) were deposited by co-evaporation to a thickness of 5 nm to form the first phosphorescent layer (the second light-emitting layer 113 b)-1 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dmdppr-dmp)₂(acac)] was 0.5:0.5:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(dmdppr-dmp)₂(acac)]), and then 2mDBTBPDBq-II, PCBBiF, and bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)) (abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula (viii) were deposited by co-evaporation to a thickness of 20 nm to form the second phosphorescent layer (the second light-emitting layer 113 b)-2 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(tBuppm)₂(acac)]). Thus, the second light-emitting layer 113 b that was the phosphorescent layer was formed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in the phosphorescent layer. This emission wavelength overlaps absorption bands on the longest wavelength sides of [Ir(dmdppr-dmp)₂(acac)] and [Ir(tBuppm)₂(acac)], so that energy transfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in the fluorescent layer (the first light-emitting layer 113 a) is higher than the singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescent substance. In addition, the triplet excitation energy of cgDBCzPA is lower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in the phosphorescent layer (the first light-emitting layer 113 a), regeneration of a singlet exciton associated with triplet-triplet annihilation and light emission are obtained easily.

After that, on the second light-emitting layer 113 b that was the phosphorescent layer, 2mDBTBPDBq-II was deposited to a thickness of 10 nm, and bathophenanthroline (abbreviation: BPhen) represented by Structural Formula (ix) was deposited to a thickness of 15 nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102 functioning as a cathode. Through the above-described steps, the light-emitting element 2 of this example was fabricated.

Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method.

(Method for Fabricating Light-Emitting Element 3)

The light-emitting element 3 was fabricated in a manner similar to that of the light-emitting element 2 except that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dmdppr-dmp)₂(acac)] that were used for forming the first phosphorescent layer (the second light-emitting layer 113 b)-1 in the light-emitting element 2 was set to 0.2:0.8:0.05 and the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] that were used for forming the second phosphorescent layer (the second light-emitting layer 113 b)-2 was set to 0.9:0.1:0.05.

In a glove box under a nitrogen atmosphere, the light-emitting element 2 and the light-emitting element 3 were each sealed with a glass substrate so as not to be exposed to the air (specifically, a sealant was applied onto an outer edge of the element, and at the time of sealing, UV treatment was performed first and then heat treatment was performed at 80° C. for 1 hour). Then, characteristics of the light-emitting elements were measured. Note that the measurement was performed with an integrating sphere at room temperature (in the atmosphere kept at 25° C.). Table 2 shows values of the characteristics at a current density of 2.5 mA/cm³.

TABLE 2 Correlated General Color Color Power External Volt- Tem- Rendering Effi- Quantum age perature Index ciency Efficiency (V) (K) duv Ra (lm/W) (%) Light- 3.0 4710 0.0142 86 36 14 Emitting Element 2 Light- 3.0 2950 0.0083 88 45 18 Emitting Element 3

Although the light-emitting elements 2 and 3 did not have a special outcoupling structure, the light-emitting elements 2 and 3 had high external quantum efficiency and power efficiency. In addition, the voltage was as low as 3 V as compared with a tandem light-emitting element.

FIG. 19A shows an emission spectrum of the light-emitting element 2, and FIG. 19B shows an emission spectrum of the light-emitting element 3. As can be seen from the emission spectra, red light emission originating from [Ir(dmdppr-dmp)₂(acac)], green light emission originating from [Ir(tBuppm)₂(acac)], and blue light emission originating from 1,6mMemFLPAPrn were observed. This indicates that light emission was sufficiently obtained from both of the first light-emitting layer 113 a that was the fluorescent layer and the second light-emitting layer 113 b that was the phosphorescent layer.

In addition, each of the light-emitting elements had a general color rendering index (Ra) of 85 or more, which means that each of the light-emitting elements had a favorable color rendering property, and had small duv; thus, the light-emitting elements are suitably used for lighting. Furthermore, the light-emitting element 2 had a color temperature of 4710 K that is day white, and the light-emitting element 3 had a color temperature of 2950 K that is incandescent color. This indicates that the light-emitting elements 2 and 3 have the characteristics conforming to the standards.

The only difference between the light-emitting element 2 and the light-emitting element 3 lies in the mixing ratio of the substances for forming the second light-emitting layer 113 b. In other words, this example reveals that white light emission in a wide color temperature range of 2950 K to 4710 K was able to be obtained by adjusting the mixing ratio of the substances, which was a simple method. Note that a color temperature of 2950 K or less or 4710 K or more can be obtained depending on adjustment. In addition, it is also a significant feature that the light emission in the wide color temperature range was obtained without a large decrease in efficiency. In this example, the case of white light emission is described because the light-emitting elements each emitting light of three colors of blue, green, and red were fabricated. In the case where a light-emitting element emitting light of another color is fabricated, a mixed color ratio of light emission can be controlled and a desired emission color can be obtained easily by adjusting the mixing ratio of substances included in the light-emitting element.

As described above, the light-emitting element 2 and the light-emitting element 3 had highly well-balanced, favorable characteristics and can be fabricated easily and inexpensively. The above-described results were attributed to the following: diffusion of excitons was suppressed and non-radiative decay of the triplet excitation energy was reduced by using the exciplex as an energy donor of the phosphorescent layer, and the emission efficiency was improved because of occurrence of delayed fluorescence due to triplet-triplet annihilation in the host material in the fluorescent layer.

Example 3

In this example, a method for fabricating a light-emitting element 4 of one embodiment of the present invention and characteristics thereof are described. In the light-emitting element 4, the first light-emitting layer 113 a was formed on the cathode side and the second light-emitting layer 113 b was formed on the anode side. Structural formulae of organic compounds used for the light-emitting element 4 are shown below.

(Method for Fabricating Light-Emitting Element 4)

A film of indium tin oxide containing silicon oxide (ITSO) was formed to a thickness of 110 nm over a glass substrate by a sputtering method to form the first electrode 101. The electrode area was 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over the substrate, a surface of the substrate was washed with water and baked at 200° C. for 1 hour, and then UV ozone treatment was performed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10⁻⁴ Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to a substrate holder provided in the vacuum evaporation apparatus so that the surface on which the first electrode 101 was formed faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. After that, on the first electrode 101, 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxide were deposited by co-evaporation by an evaporation method using resistance heating to form the hole-injection layer 111. The thickness was set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2 (=DBT3P-II: molybdenum oxide).

Next, on the hole-injection layer 111, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula (vi) was deposited to a thickness of 20 nm to form the hole-transport layer 112.

On the hole-transport layer 112, 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v), PCBBiF, and (dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)]) represented by Structural Formula (vii) were deposited by co-evaporation to a thickness of 20 nm to form the first phosphorescent layer (the second light-emitting layer 113 b)-1 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tppr)₂(dpm)] was 0.2:0.8:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(tppr)₂(dpm)]), and then 2mDBTBPDBq-II, PCBBiF, and bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)) (abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula (viii) were deposited by co-evaporation to a thickness of 5 nm to form the second phosphorescent layer (the second light-emitting layer 113 b)-2 so that the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was 0.3:0.7:0.05 (=2mDBTBPDBq-II: PCBBiF: [Ir(tBuppm)₂(acac)]). Thus, the second light-emitting layer 113 b was that was a phosphorescent layer was formed. After that, 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (abbreviation: cgDBCzPA) represented by Structural Formula (iii) and N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv) were deposited by co-evaporation to a thickness of 25 nm to form the first light-emitting layer 113 a that was a fluorescent layer so that the weight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.04 (=cgDBCzPA: 1,6mMemFLPAPrn). Through the above-described steps, the light-emitting layer 113 was formed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in the second light-emitting layer 113 b that is the phosphorescent layer. This emission wavelength overlaps absorption bands on the longest wavelength sides of [Ir(tppr)₂(dpm)] and [Ir(tBuppm)₂(acac)], so that energy transfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in the fluorescent layer (the first light-emitting layer 113 a) is higher than the singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescent substance. In addition, the triplet excitation energy of cgDBCzPA is lower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in the phosphorescent layer (the first light-emitting layer 113 a), regeneration of a singlet exciton associated with triplet-triplet annihilation and light emission are obtained easily.

After that, on the first light-emitting layer 113 a that was the fluorescent layer, cgDBCzPA was deposited to a thickness of 10 nm, and bathophenanthroline (abbreviation: BPhen) represented by Structural Formula (ix) was deposited to a thickness of 15 nm to form the electron-transport layer 114.

After the formation of the electron-transport layer 114, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm to form the electron-injection layer 115. Finally, aluminum was deposited by evaporation to a thickness of 200 nm to form the second electrode 102 functioning as a cathode. Through the above-described steps, the light-emitting element 1 of this example was fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method.

An element structure of the light-emitting element 4 is shown in Table 3.

In a glove box under a nitrogen atmosphere, the light-emitting element 4 was sealed with a glass substrate so as not to be exposed to the air (specifically, a sealant was applied onto an outer edge of the element, and at the time of sealing, UV treatment was performed first and then heat treatment was performed at 80° C. for 1 hour). Then, characteristics of the light-emitting element 4 were measured at a current density of 2.5 mA/cm² and at around 1000 cd/cm².

TABLE 3 Correlated General Color Color Power External Tem- Rendering Effi- Quantum Voltage perature Index ciency Efficiency (V) (K) duv Ra (lm/W) (%) Light- 2.7 2690 0.01 84 29 11 Emitting Element 4

Although the light-emitting element 4 did not have a special outcoupling structure, the light-emitting element 4 had high external quantum efficiency and power efficiency. In addition, the light-emitting element 4 had a very low voltage of 2.7 V as compared with a tandem light-emitting element.

FIG. 20 shows an emission spectrum of the light-emitting element 4. As can be seen from the emission spectrum, red light emission originating from [Ir(tppr)₂(dpm)], green light emission originating from [Ir(tBuppm)₂(acac)], and blue light emission originating from 1,6mMemFLPAPrn were observed. This indicates that light emission was sufficiently obtained from both of the first light-emitting layer 113 a that was the fluorescent layer and the second light-emitting layer 113 b that was the phosphorescent layer.

In addition, the light-emitting element 4 had a general color rendering index (Ra) of 84, which means that the light-emitting element 4 had a favorable color rendering property, and had small duv; thus, the light-emitting element 4 is suitably used for lighting. Furthermore, the light-emitting element 4 had a color temperature of 2690 K that is incandescent color. This indicates that the light-emitting element 4 had the characteristic conforming to the standards.

As described above, the light-emitting element 4 had well-balanced, favorable characteristics and was able to be fabricated easily and inexpensively. The above-described results were attributed to the following: diffusion of excitons was suppressed and non-radiative decay of the triplet excitation energy was reduced by using the exciplex as an energy donor of the phosphorescent layer, and the emission efficiency was improved because of occurrence of delayed fluorescence due to triplet-triplet annihilation in the host material in the fluorescent layer. It was also found that favorable characteristics were able to be obtained even when the stacking order in the light-emitting layer 113 was changed.

REFERENCE NUMERALS

101: first electrode, 102: second electrode, 103: EL layer, 111: hole-injection layer, 112: hole-transport layer, 113: light-emitting layer, 113 a: first light-emitting layer, 113 b: second light-emitting layer, 114: electron-transport layer, 115: electron-injection layer, 400: substrate, 401: first electrode, 403: EL layer, 404: second electrode, 405: sealant, 406: sealant, 407: sealing substrate, 412: pad, 420: IC chip, 501: first electrode, 502: second electrode, 511: first light-emitting unit, 512: second light-emitting unit, 513: charge-generation layer, 601: driver circuit portion (source line driver circuit), 602: pixel portion, 603: driver circuit portion (gate line driver circuit), 604: sealing substrate, 605: sealant, 607: space, 608: wiring, 609: flexible printed circuit (FPC), 610: element substrate, 611: switching FET, 612: current control FET, 613: first electrode, 614: insulator, 616: EL layer, 617: second electrode, 618: light-emitting element, 623: n-channel FET, 624: p-channel FET, 625: drying agent, 901: housing, 902: liquid crystal layer, 903: backlight unit, 904: housing, 905: driver IC, 906: terminal, 951: substrate, 952: electrode, 953: insulating layer, 954: partition layer, 955: EL layer, 956: electrode, 1001: substrate, 1002: base insulating film, 1003: gate insulating film, 1006: gate electrode, 1007: gate electrode, 1008: gate electrode, 1020: first interlayer insulating film, 1021: second interlayer insulating film, 1022: electrode, 1024W: first electrode of light-emitting element, 1024R: first electrode of light-emitting element, 1024G: first electrode of light-emitting element, 1024B: first electrode of light-emitting element, 1025: partition, 1028: EL layer, 1029: second electrode of light-emitting element, 1031: sealing substrate, 1032: sealant, 1033: transparent base material, 1034R: red coloring layer, 1034G: green coloring layer, 1034B: blue coloring layer, 1035: black layer (black matrix), 1036: overcoat layer, 1037: third interlayer insulating film, 1040: pixel portion, 1041: driver circuit portion, 1042: peripheral portion, 2001: housing, 2002: light source, 3001: lighting device, 5000: display region, 5001: display region, 5002: display region, 5003: display region, 5004: display region, 5005: display region, 7101: housing, 7103: display portion, 7105: stand, 7107: display portion, 7109: operation key, 7110: remote controller, 7201: main body, 7202: housing, 7203: display portion, 7204: keyboard, 7205: external connection port, 7206: pointing device, 7210: second display portion, 7301: housing, 7302: housing, 7303: joint portion, 7304: display portion, 7305: display portion, 7306: speaker portion, 7307: recording medium insertion portion, 7308: LED lamp, 7309: operation key, 7310: connection terminal, 7311: sensor, 7401: housing, 7402: display portion, 7403: operation button, 7404: external connection port, 7405: speaker, 7406: microphone, 7400: mobile phone, 9033: clip, 9034: switch, 9035: power switch, 9036: switch, 9038: operation switch, 9630: housing, 9631: display portion, 9631 a: display portion, 9631 b: display portion, 9632 a: touchscreen region, 9632 b: touchscreen region, 9633: solar cell, 9634: charge and discharge control circuit, 9635: battery, 9636: DC-to-DC converter, 9637: operation key, 9638: converter, and 9639: button.

This application is based on Japanese Patent Application serial no. 2013-174560 filed with the Japan Patent Office on Aug. 26, 2013, Japanese Patent Application serial no. 2013-249449 filed with the Japan Patent Office on Dec. 2, 2013, and Japanese Patent Application serial no. 2014-112119 filed with the Japan Patent Office on May 30, 2014, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A light-emitting device comprising: an anode; a first light-emitting layer; a second light-emitting layer; and a cathode in this order, wherein the second light-emitting layer comprises a first substance which is capable of converting triplet excitation energy into luminescence, a first organic compound, and a second organic compound, wherein the first organic compound and the second organic compound are configured to form an exciplex, wherein a peak wavelength of light emitted from the first light-emitting layer is shorter than a peak wavelength of light emitted from the second light-emitting layer, wherein an energy value of peak wavelength of an emission spectrum of the exciplex is a first energy value, wherein an energy value of a peak wavelength of a lowest-energy-side absorption band of the first substance is a second energy value, wherein a difference between the first energy value and the second energy value is smaller than 0.2 eV, wherein the first organic compound comprises a first carbazole skeleton, and wherein the second organic compound comprises a second carbazole skeleton.
 2. A lighting device comprising: a first light-emitting device; a second light-emitting device; a third light-emitting device; and a fourth light-emitting device, wherein each of the first light-emitting device, the second light-emitting device, the third light-emitting device, and the fourth light-emitting device comprises the light-emitting device according to claim 1, wherein the first light-emitting device overlaps with a red color filter, wherein the second light-emitting device overlaps with a green color filter, wherein the third light-emitting device overlaps with a blue color filter, and wherein the fourth light-emitting device comprise a region which overlaps with none of the red color filter, the green color filter, and the blue color filter.
 3. The light-emitting device according to claim 1, wherein the fluorescent substance comprises a pyrenediamine compound.
 4. A lighting device comprising: a first light-emitting device; a second light-emitting device; a third light-emitting device; and a fourth light-emitting device, wherein each of the first light-emitting device, the second light-emitting device, the third light-emitting device, and the fourth light-emitting device comprises the light-emitting device according to claim 3, wherein the first light-emitting device overlaps with a red color filter, wherein the second light-emitting device overlaps with a green color filter, wherein the third light-emitting device overlaps with a blue color filter, and wherein the fourth light-emitting device comprise a region which overlaps with none of the red color filter, the green color filter, and the blue color filter.
 5. The light-emitting device according to claim 1, wherein the first light-emitting layer comprises a fluorescent substance and a host material.
 6. The light-emitting device according to claim 5, wherein a triplet excitation level (a T1 level) of the host material is lower than a T1 level of the fluorescent substance. 